Combustible gas detection instruments in use in industry today commonly employ a Wheatstone bridge configuration in the conventional manner by modifying two elements of the bridge. For example, one element would be a conventional refractory catalytic detector containing platinum whereby the combustible gas is burned, heating and increasing the resistance of the catalytic element, and the other element would be a refractory containing a poisoning element so that the combustible gas would not react. In this manner, a signal would be developed in the conventional manner as shown in FIG. 1.
A shortcoming of this type of construction is that the detecting elements contain a finite amount of catalytic material which can be inhibited with excessive exposure to combustible gases, such as methane. In the case of portable instruments, while it is not expected that they will be exposed to high levels of combustible gas (considering that they are usually portable and hand-held), nevertheless, at times, it is expected that the detector will be exposed to these high levels; therefore, the design of the detector should be adequate to withstand these high levels, maintaining stability at concentrations, for example, of 75% to 100% LEL (3.75 to 5% Vol. methane).
What happens during these high levels of combustible gas is that the burning gas reacts with air (oxygen) and the products of combustion are generally water and CO.sub.2. During the high levels of gas combustion, the molecules are breaking down and uniting with air to ultimately form water and carbon dioxide, but intermediate compounds are the formation of hydroxides on the surface, these hydroxides temporarily uniting with the meta-stable alumina refractory and/or catalytic compounds. For example, in the following equation, during the burning of the combustible gas right at the surface of the catalyst, the very active state of the compounds may form temporary hydroxides (hydroxylation) on the active sites which inhibit the catalytic activity; ##STR1## The metal hydroxide compounds, or the hydroxide group forming at the refractory-carrier-catalyst active site, may blind or otherwise inhibit the activity of the site. At high methane concentrations, (2.5% to 5% Vol.), the reaction rate favoring the formation of such compounds at the active sites can increase. Upon removal of the high methane concentrations and exposure to clean air, the hydroxide groups leave and re-expose the active catalytic sites (dehydroxylation).
This can also be proven by exposing the detector to high concentrations of methane, for example, 90% Vol., when little or no burning occurs and the equilibrium then shifts to reducing and removal of the hydroxylated sites with formation of water, thus re-exposing the active catalytic site.
A similar action occurs, according to British Pat. No. 1,604,081, wherein advantage is tken of this dehydroxylation phenomenon by raising the temperature of the refractory, removing the hydroxide groups, generating active refractory catalytic sites, cooling, followed by immediate application of the catalyst within minues so that the catalyst gets to the active site before water molecules from the air, producing more and more stable catalytic sites rather than a site being jointly competed for by a water molecule and a catalyst atom.
It is evident that a large area meta-stable alumina refactory alone is not a sufficient catalyst for this application, nor are any noble metals (Pt, Rb, etc.) in the massive solid state. Rather, the catalytic noble metal must be dispersed (electronic bonds strained) on the large area refractory, forming a carrier-catalyst active site to produce suitably active catalysts.
One method of reducing the diffusion of the gas and what is employed sometimes in industry, is to make the dust shield finer, but this leads to easy plugging.
Another method of reducing the diffusion rate of the gas to the catalyst and sometimes employed industrially, is to coat the catalyst bead with further coats of non-reactive refractory. However, this method poses several thermal conductivity and thermal radiation problems tending to reduce the temperature of the catalytic bead non-uniformly with given power, toincrease power requiements, and, also, to introduce non-linearity problems as the temperature of the bead changes.